The present invention relates generally to radar surveillance systems, and more specifically to a tristatic target location system which uses tristatic Doppler frequency shift and angle of arrival to locate radar targets and determine their trajectory.
Many target location techniques have been applied which use measurable characteristics of signals scattered by an object to locate the object in space. Conventional radars and active sonar systems, for example, use the measured time delay and angle of arrival of a single scattered signal to determine the range and bearing from the surveillance device to the target. Bistatic radars, in which the source of the signal and the receiver are some distance apart, also locate targets from the time delay and angle of arrival of the scattered signal, although the interpretation of measurements is somewhat more complicated. These approaches to target location depend on measurements of time delay. This dependence on time delay data imposes constraints on the types of signals which can be used, and on the design of the receiving equipment.
An accurate and unambiguous measurement of time delay requires: a broad band signal waveform; a signal that exhibits an unambiguous autocorrelation function over the time delays of interest; and time synchronization between the transmitter and receiver
These requirements are difficult to achieve, particularly if the signal to be used is not under the control of the surveillance system designer Accurate measurements of Doppler frequency (time rate of change of signal delay), on the other hand, can often be obtained on signal waveforms which do not support useful time delay measurements. The Doppler shift of a scattered signal containing strong narrow band components, in particular, can be readily derived through straight forward spectral analysis. For this reason, an approach to target location which does not require time delay measurements offers some advantages.
A monostatic radar tracking system has its radar transmitter and its radar receiver located in the same physical location. A bistatic radar system has a single radar transmitter which is remotely located from a single radar receiver A tristatic radar tracking system has two radar transmitters which are remotely located from a passive radar receiver. The task of designing a tristatic radar tracking system is alleviated to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,812,493 issued to Afendykiw et al; PA1 U.S. Pat. No. 3,939,475 issued to Lewis; PA1 U.S. Pat. No. 3,991,418 issued to Bennett; PA1 U.S. Pat. No. 3,996,590 issued to Hammack; and PA1 U.S. Pat. No. 4,499,468 issued to Montana et al.
Lewis discloses a method of measuring range with a large bistatic angle radar. The system of this patent includes a target, a first transmitting station, a second transmitting station, and a receiving station. Distance to the target is determined by known transmitter and receiver distances in combination with the measured time duration.
In Bennett the direction of skywave transmissions is determined by the application of monopulse direction finding techniques by the isolation of energy in Doppler shift modes involving high resolution spectrum analyses.
Montana et al determine range using a multistage radar system with a plurality of radar stations located in a square grid pattern. In the system of this patent a computer applies a range difference similarity test, a uniqueness test, and a position test to determine real targets from the plurality of echo returns received Hammack detects and tracks moving objects using a plurality of stations. Afendykiw et al are concerned with a target position system using cross correlation techniques. A target is located in the system of this patent by combining range and angular information.
While the above-cited references are instructive, a need remains to provide a Doppler shift tristatic arrangement in which the tristatic Doppler frequency shift and angle of arrival is used to instantaneously derive the parameters of the target's trajectory through space. The present invention is intended to satisfy that need.